Mobile telecommunications systems based on one or more cellular networks are known in many different forms. A common aspect of older 2G systems (such as GSM—Global System for Mobile Communications) as well as more recent 3G (e.g. UMTS—Universal Mobile Telecommunications System) and 4G (e.g. EPS/LTE—Evolved Packet System/Long Term Evolution) systems is the need for paging of terminal devices in the system. Such terminal devices are for instance referred to as mobile stations (MS) in GSM and user equipment (UE) in UMTS and LTE, and are typically used by human users in a truly mobile manner.
Also, terminal devices are known which typically do not involve human users and which mostly have a more stationary installation. Examples are terminal devices for MTC (Machine Type Communication) or M2M (Machine To Machine) communication in EPS/LTE or UMTS. Such terminal devices are referred to as machine devices (MD).
For the purpose of this document, the term “wireless device” or just “device” will be used as a common reference to a terminal device, not limited to any particular type, which accesses a cellular network over a wireless interface. As specific examples of such devices, UEs and MDs for an EPS/LTE system will primarily be used without any prejudicial limiting effect. FIG. 1 illustrates an EPS/LTE compliant telecommunications system, which will be described in more detail in the Detailed Description section of this document, and a terminal device in the form of a UE used therein.
The purpose of the paging concept is to reach an idle UE to deliver data (signaling data or user data) in the downlink direction from the network to the UE. In order to reach the UE, the page is transmitted in the entire area where the UE may be located (according to the knowledge of the network). When moving between such areas, the UE has to inform the network, so that the network knows in which area to page the UE, when needed. Such an area typically consists of multiple cells (but less than the entire network area). This principle is based on a trade-off between the signaling (radio) resources used for paging and the signaling (radio) resources used when UEs report their location (i.e. area) to the network. In EPS/LTE, a registration area is defined as a set of Tracking Areas (TAs). Each attached UE has a list of Tracking Area Identities (TAIs), representing the UE's current registration area (i.e. set of TAs), which is stored in the UE and the MME (Mobility Management Entity) in which the UE is currently registered.
To be reachable for paging an idle UE has to monitor a certain repetitive downlink signaling channel to check for paging indications directed towards it. In EPS/LTE, this consists of monitoring the PDCCH (Physical Downlink Control Channel) for downlink resource assignments addressed to a paging RNTI (P-RNTI, Paging Radio Network Temporary Identifier). The P-RNTI is shared among many UEs (potentially all, as is the case in LTE). When detecting such a paging indication, the UE has to receive a Paging RRC (Radio Resource Control) message, which is transmitted on the downlink transmission resources on the PDSCH (Physical Downlink Shared Channel) that were assigned by the paging indication on the PDCCH. This Paging RRC message contains the identity or identities of the UE(s) that the paging concerns and which is/are thus requested to contact the network. When finding its identity in a Paging RRC message, the UE initiates a random access procedure towards the eNB (E-UTRAN (Evolved Universal Terrestrial Radio Access Network) Node B), establishes an RRC connection with the eNB and sends a Service Request NAS (Non Access Stratum) message to the MME (via the eNB).
As an energy saving feature, primarily to promote long battery lifetimes, a paging DRX (Discontinuous Reception) mechanism is used. This allows the UE to spend most of its time in a more energy-efficient mode, e.g. a “sleep mode”, and activate its receiver for the purpose of monitoring the PDCCH for paging indications only on specific occasions (also denoted active periods). The UE and the eNB have a common understanding of which these occasions are, so that the eNB can send paging indications concerning the UE when the UE listens. A paging DRX cycle is thus divided into a sleep period and an active period, wherein the active period is essentially equal to a potential paging occasion.
Currently, the paging DRX cycle is defined by the paging configuration parameters T and nB together with IMSI modulo 1024. These parameters are input to an algorithm which points out the frames (within each SFN (System Frame Number) cycle) and subframes (within these frames) in which the UE may be paged (i.e. the paging occasions).
Details of the algorithm for calculating the frames and subframes where pages may occur, i.e. the paging occasions, according to the current LTE standard is specified in chapter 7 of 3GPP TS 29.274 V11.2.0, “3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 (Release 11)”, March 2012. [1]
The T parameter is defined as the minimum of the defaultPagingCycle IE (Information Element), which is broadcasted in SIB2 (System Information Block Type 2) of the system information, and a possible pre-configured UE-specific DRX cycle length. If a UE-specific DRX cycle is used, the UE sends it to the MME in the Attach Request NAS message, when attaching to the network, and/or in a Tracking Area Update Request NAS message. The nB parameter is broadcasted together with the defaultPagingCycle IE in SIB2 of the system information. The IMSI (International Mobile Subscriber Identity) of the UE is stored in the USIM (Universal Subscriber Identity Module) on the UICC (Universal Integrated Circuit Card) in the UE as well as in the MME in which the UE is registered.
FIG. 2 gives a schematic overview of the different steps performed when a UE 250 is paged in an EPS/LTE system 200, like the one illustrated in the aforementioned FIG. 1. In EPS/LTE, paging is initiated from the core network, typically triggered by arrival of downlink user data through a PGW (PDN GW; Packet Data Network Gateway) 210. The user data to be delivered is buffered in a SGW (Serving Gateway) 220, which sends a Downlink Data Notification GTPv2-C (GPRS (General packet radio service) Tunnelling Protocol version 2) message 212 to an MME 230. For details of this message, see reference [1] above.
The MME 230 checks which eNBs 240 have cells belonging to any of the TAs whose TAIs are included in the UE's current TAI list, and then sends a paging control message 232 in the form of a PAGING S1AP (S1 Application Protocol) message across the S1 interface 270 to each of these eNBs, requesting the respective eNB 240 to page the UE in all cells belonging to any of the concerned TAs, i.e. all cells having a TAI that is included in the UE's list of TAIs. The PAGING S1AP message includes IMSI modulo 1024 in the UE Identity Index Value IE, the UE's list of TAIs in the List of TAIs IE and a possible UE-specific DRX cycle length in the Paging DRX IE. Details of the PAGING S1AP message are found in 3GPP TS 36.413 V10.5.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) (Release 10)”, March 2012. [2]
Upon receiving the paging control message PAGING S1AP 232 from the MME 230, the eNB 240 calculates the applicable paging occasions and awaits the next paging occasion. Then, at the next paging occasion, the eNB 240 sends a paging request on the PDSCH in the form of a Paging RRC message 244, indicated by a downlink resource assignment addressed to a paging RNTI (P-RNTI) on the PDCCH as seen at 242, to the UE 250 in the subframe corresponding to the paging occasion. (It is to be noticed that even though 242 and 244 have been indicated as separate events in the schematic illustration in FIG. 2, they take place concurrently in the same subframe.) The eNB 240 does this for each of its cells that take part in the paging procedure. Details of the Paging RRC message are found in 3GPP TS 36.331 V10.5.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 10)”, March 2012.
As already mentioned above, when the UE 250 finds its identity in the Paging RRC message 244, the UE 250 initiates a random access procedure towards the eNB 240, establishes an RRC connection with the eNB 240 and sends a paging response 246 in the form of a Service Request NAS message to the MME 230.
Since paging is performed in multiple cells, but the UE 250 only listens to transmissions in a single cell, the paging procedure inevitably results in a waste of transmission resources, and thus also unnecessarily increased interference. Many paging requests (paging indications 242 and Paging RRC messages 244) as well as many paging control messages (PAGING S1AP messages 232) will be redundant.
A currently popular vision of the future development of the communication in cellular networks comprises huge numbers of small autonomous devices, which typically and more or less infrequently (e.g. once per week to once per minute) transmit and receive only small amounts of data (or are polled for data). These devices are assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers (which configure the devices and receive data from them) within or outside the cellular network. As mentioned above, this type of communication is often referred to as M2M or MTC communication, and the devices are referred to as MDs (or M2M or MTC devices). With the nature of MDs and their assumed typical uses follow that they will often have to be very energy-efficient, since external power supplies will often not be available. Instead they have to sustain on energy harvesting or batteries, and it would not be practically or economically feasible to frequently replace or recharge their batteries.
One way of achieving low energy consumption in MDs is to use long (extended) paging DRX cycles. This will allow the MDs to spend the vast majority of their idle mode time in an energy-efficient sleep mode with the wireless receiver turned off. When extended paging DRX cycles are introduced, which potentially are much longer than current paging DRX cycles, they will span across multiple SFN cycles (e.g. N cycles). To facilitate that this works smoothly in combination with the current relaxed SFN cycle synchronization (i.e. the lack of synchronization requirement between cells), extended paging DRX cycles are likely to be defined as multiples of SFN cycles. For the purpose of this document an SFN cycle in which a UE may be paged according to its extended paging DRX cycle may be denoted as a paging SFN cycle. It may further be assumed that within a paging SFN cycle, the frame(s) and subframe(s) in which to page the UE will be derived from one or more system information parameter(s), which may be different in different cells, i.e. similar to the current paging DRX scheme. Furthermore, it is very likely that the maximum value of T (and thus also the maximum values of the defaultPagingCycle IE and a possible pre-configured UE-specific DRX cycle length) will be increased from 256 to 1024 radio frames, i.e. to 10.24 seconds, and/or that one or more other paging configuration parameter(s) and another algorithm for calculating paging occasions will be standardized for extended paging DRX cycles.
This should preferably result in that only a single paging occasion occurs during the paging SFN cycle, i.e. one paging occasion every Nth SFN cycle, when extended paging DRX spanning multiple (i.e. N) SFN cycles is used. As a side note, when used in conjunction with extended paging DRX cycles spanning multiple SFN cycles, the interpretation of the defaultPagingCycle IE and a possible pre-configured UE-specific DRX cycle length have to be changed, and possibly renamed, from indicating the length of the actual extended paging DRX cycle to being parameters that are used when deriving the frame(s) within a paging SFN cycle in which the UE may be paged (and indicating the paging DRX cycle to use within the paging SFN cycle in case a scheme allowing paging a UE in multiple frames within the same paging SFN cycle is used).
As can be understood from the above, a general problem with the paging procedure is that it is inherently inefficient in terms of resource usage, since it wastes redundant downlink transmission resources for paging indications and Paging RRC messages (and also PAGING S1AP messages) in cells where the target UE is not listening.
Redundant paging-related radio transmissions, i.e. paging indications on the PDCCH as well as Paging RRC messages, also have other disadvantages than wasting resources. One such disadvantage is that they increase both inter-cell and intra-cell interference. Another disadvantage is that UEs that are not actually targeted by the page have to receive redundant Paging RRC messages, thus wasting resources, e.g. in the form of battery drainage, in the UEs as well. Yet another disadvantage is that the redundant radio transmissions will waste energy, thus keeping the energy consumption of the network unnecessarily high.
Certain attempts have been made by proposing various staged/phased paging schemes, wherein the paging of a UE is divided into two or more stages or phases. The UE is first paged in a certain part of its registration area (i.e. a subarea), and only if no response from the UE is received in this first step, will the UE be paged in the remainder of its registration area. This may possibly be divided into further stages or phases with different paging areas (distinct or overlapping) used for each stage or phase. The choice of subarea(s) to page the UE in and the order in which to use the subarea(s) are based on additional information in the form of location knowledge and/or historical data, such as the UE's latest known cell, possibly combined with knowledge about movement direction and/or speed, or historical statistics of the UE's (or the user's) most commonly visited cell(s).
While such staged/phased schemes may reduce the redundant paging-related radio transmissions and the negative effects thereof, it will introduce significant additional delay when the UE does not respond to the paging attempt in the first subarea. The more stages/phases and subareas the scheme uses, the less time-efficient will the procedure become, since each stage/phase adds the delay of having to wait for the conclusion that no response will be received in the previous stage/phase. A considerable part of the delay is caused by the paging DRX, which contributes with up to 2.56 seconds in each paging stage/phase with the currently specified longest paging DRX cycle. With extended paging DRX cycles the delay contribution will be greatly magnified.
Also, staged/phased paging schemes require network resources to provide, collect or analyze the additional information referred to above.
There is thus a need for improvements of the paging control in known telecommunications systems.